Inertial navigation sensors play a very important role in today's technology. Three axis gyroscopes and accelerometers are widely used for tracking the movement or velocity of any subject, such as an object, vehicle or person. On top of these sensors (e.g. one or more 3-axis gyroscopes and one or more 3-axis accelerometers), initial position of the subject is also acquired from the Global Positioning System (GPS) to find and track the subject around the Earth. However, the initial orientation of the sensor is required for tracking the position of the subject and this information can be obtained by a three axis magnetometer sensor. By measuring the strength and direction of the Earth's magnetic field, a magnetometer system can detect its orientation with respect to the Earth. Therefore, magnetometers are indispensable elements for tracking systems.
In addition to sensing orientation with respect to the Earth, magnetometers have wide range of usage. These application areas can be listed as automotive sensors, non-destructive material testing, security systems, structural stability, medical sensors, and military instruments, for example. To meet the requirements of all these different applications, there are many different magnetic field sensors available. Some of the magnetic field sensors can be listed as Superconducting Quantum Interference Device (SQUID), Hall Effect Magnetometers, search coils, Fluxgate Magnetometers, Anisotropic Magnetoresistive (AMR), Giant Magnetoresistive (GMR), Fiber Optic sensors and MEMS (Micro-Electro-Mechanical-System) Magnetometers.
FIG. 1A compares the resolution and maximum range of different magnetic field measuring technologies. Based on FIG. 1A, search coil and SQUID sensors have the largest measurement range to resolution ratio. However, search coils can only measure varying magnetic fields which is not suitable for measuring the Earth's magnetic field, while SQUID sensors need special operation environment (e.g. liquid Helium supply, electromagnetic shielding) and have a power consumption of several watts. MEMS magnetometers also have a large measurement range to resolution ratio, but their real advantage is having a small size and low power consumption. They do not need any specific magnetic material to operate and they are suitable for many application areas. FIG. 1B shows the measurement range requirement for magnetometers for different application areas.
There are different MEMS capacitive magnetometers available, including a lateral axis magnetometer with a see-saw structure, and vertical axis magnetometers with capacitive comb finger sensing. Capacitive MEMS magnetometers basically carry a sinusoidal current orthogonal to their sensitive axis. An externally applied magnetic field generates a force proportional to both the magnitude of the current and the length of the current carrying beam and the direction of the force is in the direction of the cross product of the current and magnetic field, which is the sensitive axis of the magnetometer.
Generally, a current is applied to a magnetometer so that with an external magnetic field, a Lorentz force is generated on the magnetometer. In conventional magnetometers, these structures drive the current through the suspended structure to generate the Lorentz Force when there is an applied magnetic field. The current is very inefficiently used as the current passes through the structure just once. Further, the suspended structure itself is a high resistivity current path.